Terephthalic acid is produced by oxidation of p-xylene in the presence of a catalyst and a promoter in an aliphatic carboxylic acid as a fluid medium. From the process optimization point of view, it is crucial to have the aliphatic carboxylic acid recovered and recycled without affecting the quality of final product.
Terephthalic acid is useful in a variety of industrial applications and chemical processes mainly in polyester field. Terephthalic acid is produced commercially in an oxidation reactor by oxidation of p-xylene using oxygen in acetic acid as a fluid medium, and in the presence of a catalyst and a promoter. Salts of Cobalt, Manganese, Chromium, Copper, Nickel, Vanadium, Iron, Molybdenum, Tin, Cerium, Zirconium, Cesium and Titanium such as acetate or bromide are used as catalyst and organic and inorganic bromide compounds such as hydrobromic acid and ionic liquids containing bromide as anion are used as the promoters in the reaction.
Water is produced during this reaction. Further, other organic compounds like trimellitic acid, 4-carboxybenzaldehyde, p-toluic acid, brominated compounds and the like are produced during the oxidation reaction. Water and these byproducts are present in soluble form in the reaction mixture.
First step of downstream process comprises a series of crystallizers used for crystallizing out terephthalic acid from the reaction mixture. The precipitated terephthalic acid is obtained as a cake. The mother liquor obtained after crystallization contains terephthalic acid, oxidation intermediates of terephthalic acid, other organic chemicals such as side products, byproducts, catalysts and water dissolved in acetic acid.
One byproduct of the oxidation reaction is trimellitic acid. Trimellitic acid, at concentrations above 2000 ppm, forms manganese trimellitate and similar salts with other metals which forms a coat on the filter cloth and affect filtration system performance. The other organic chemicals in this stream include isophthalic acid, o-phthalic acid, benzoic acid, other brominated organic acids, 4-carboxybenzaldehyde, p-toluic acid, salt/s of metal/s. High concentrations of these impurities impact both the quality and the utilization of the plant due to fouling in the vessel and in the circulation heater that requires frequent caustic washes.
For maintaining lower concentration of these organic compounds, the filtrate from the crystallization step is divided into two parts. One part (80-95%) is recycled directly to the oxidation process, whereas, the second stream (5-20%), called the purge stream, is sent for purification before it is recycled. The step of purification involves removal of water and undesired chemicals form the purge stream.
The purge stream is sent to the fluid medium recovery area where it is heated at high temperature and flashed for recovery of acetic acid. High temperature flashing and evaporation is a highly energy intensive process.
After removal of most of the acetic acid, valuable organic chemicals like terephthalic acid, 4-carboxy benzaldehyde, p-toluic acid, benzoic acid and the like are discarded or sold out as low value product. This results in a low yield of terephthalic acid.
To make the process further economical, it is required that the terephthalic acid and oxidation intermediates are sent back to the oxidation reactor. The oxidation intermediates of terephthalic acid will get further oxidized and produce terephthalic acid which will increase yield of the process.
There is, therefore, a need to develop a process for continuous oxidation of p-xylene to terephthalic acid where the fluid medium and valuable chemicals are recovered and recycled in an energy efficient manner. Further, it is desired that the water and trimellitic acid produced during the oxidation are continuously removed.